1. Field of the Invention
2. Description of the Prior Art
Traditional power supplies with a simple diode rectifier draw highly nonlinear current that pollutes the utility or power circuits. In order to reduce the current harmonics and meet the EN61000-3-2 regulation, many approaches have been proposed. Active two-stage Power factor Correction (PFC) and single-stage PFC, active power filters are typical ways to reduce the harmonics and improve the power factor. For small power supplies (typical below 200 watts), single-stage PFC is attractive due to its low cost. The prior art has introduced a single-stage converter that combined a boost converter with a two-switch dc-dc converter as shown in FIG. 1. This topology employs a two-terminal input-current shaping (ICS) cell that is comprised of one auxiliary winding of transformer, an input inductor and a diode. The turns number of the auxiliary winding is exactly equal to the turns number of primary transformer winding. In other words, the turns ratio between auxiliary winding and primary winding is unity.
The equivalent circuit for the input stage of the prior art converter in FIG. 1 during each switching cycle is shown in FIGS. 2a and 2b for the on and off time equivalent circuits respectively. When the switch is on, the auxiliary transformer winding is in series with the inductor and energy storage capacitor CB. The voltage across the inductor is given by
VLin=Vg+NIVCB/Npxe2x88x92VCB=Vgxe2x80x83xe2x80x83(1)
The switch current is the sum of input inductor current and load current, which is given by
isw=NIiLin/Np+idc-dc=iLin+idc-dc
where isw is the switching current, iLin is the input inductor current and idc-dc is the current of the dc-dc converter to supply the output power. Because the turns number of the auxiliary winding is the same as the primary winding NI=Np, the voltage across the inductor is equal to the input voltage Vg. When the switch is turned off, the equivalent circuit is shown in FIG. 2b. The inductor current is discharged to energy storage capacitor. The operation is similar to a boost converter. If the inductor operates in a discontinuous conduction mode (DCM), the average input current will approximately follow the input voltage and low current distortion will result. The expected the input current is shown in FIG. 3.
In the prior art, the dc-dc converter is operated in DCM mode. However, DCM usually results in large current ripple leading to large conduction losses for both switch and output diode. For low voltage applications (such as 5 v output), the continuous conduction mode (CCM) of operation of the dc-dc converter is preferred. However, if the dc-dc converter operates in the continuous conduction mode, the energy-storage capacitor voltage VCB is strongly dependent on the load and input voltage. For universal applications, at the light load and high voltage, the dc bus voltage VCB can be as high as 1000 volts.
FIG. 4 shows the calculated capacitor voltage vs. load and input voltage for a 100 w universal input power supply for converter in FIG. 1 with a CCM operated dc-dc converter. It will require high voltage capacitor and high voltage switch, which is undesirable for industry applications due to its high cost.
What is needed is a method to realize a single-stage PFC with reduced capacitor voltage VCB.
The invention is a DC-to-DC converter coupled to an AC source comprising a rectifier bridge having an input coupled to the AC source and having an output. An input inductor is coupled to the output of the rectifier stage. An auxiliary winding is coupled to the input inductor. The auxiliary winding is characterized by a first number of turns. A storage capacitor is coupled to the auxiliary winding. An output transformer is provided with a primary winding. The primary is characterized by a second number of turns. A first switch is coupled in series with the primary of the output transformer. The first switch and primary are coupled in parallel with the storage capacitor to the auxiliary winding. The first number of turns is not equal to the second number of turns so that voltage across the storage capacitor is reduced without load carrying capacity of the DC-to-DC converter. In particular, the first number of turns is less than the second number of turns.
Alternatively, DC-to-DC converter may be characterized by a winding ratio, m, between the auxiliary winding and the primary of less than 1.0 and greater than zero. In one illustrated embodiment the winding ratio. m. is approximately 0.73.
The windings ratio has been chosen so that the storage capacitor can be rated at or less than 450 V.
The topology of the DC-to-DC converter in one embodiment may further comprise an inductor in parallel with the storage capacitor.
In another embodiment the DC-to-DC converter may further comprise a second switch coupled in series with the primary and the first switch.
The invention also includes operating a circuit of the foregoing topology and providing a circuit with the foregoing topology.
While the method has been described for the sake of grammatical fluidity as steps, it is to be expressly understood that the claims are not to be construed as limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.